This invention relates to a method of producing an oxygen sensing element for use in a device to detect the concentration of oxygen in a gas atmosphere or to detect the air/fuel ratio of a gas mixture supplied to, e.g., an internal combustion engine based on the amount of oxygen contained in the exhaust gas, which element takes the form of a lamination of relatively thin layers including an oxygen ion conductive solid electrolyte layer, a reference electrode layer laid on one side of the solid electrolyte layer and a measurement electrode layer laid on the other side.
The usefulness of oxygen sensors of the concentration cell type utilizing an oxygen ion conductive solid electrolyte as typified by ZrO.sub.2 stabilized with Y.sub.2 O.sub.3 or CaO has been well appreciated in various fields.
In the current automobile industries it has been popularized to provide an oxygen sensor of this type to the engine exhaust system to detect changes in the actual air/fuel ratio of an air-fuel mixture supplied to the engine based on the amount of oxygen contained in the exhaust gas. The oxygen-sensitive element of the sensor comprises a sintered solid electrolyte layer, a measurement electrode layer formed on one side of the solid electrolyte layer so as to be exposed to a gas subject to measurement and a reference electrode layer formed on the opposite side where a reference oxygen partial pressure is to be established. These three layers constitute an oxygen concentration cell which can generate an electromotive force between the two electrode layers depending on the magnitude of an oxygen partial pressure in the gas to which the measurement electrode layer is exposed.
A recent trend is to construct this concentration cell in the form of a lamination of thin, film-like layers. For example, the solid electrolyte layer is made as thin as about 30 microns and the two electrode layers are made still thinner. The cell of the laminated construction is mounted on a thin plate of a ceramic material, which plate is called a substrate or shield layer, such that the reference electrode layer of the cell is tightly sandwiched between the shield layer and the solid electrolyte layer. Usually the concentration cell part of this element, or the entire element, is coated with a porous protecting layer of a ceramic material.
Usually the material of the reference electrode layer is a metal such as platinum or its alloy or an electronically conducting mixture of a certain metal and its oxide, such as a Ni-NiO mixture, which can serve also as the source of a reference oxygen partial pressure. A typical material of the measurement electrode layer is platinum which acts as a catalyst or its alloy.
Each of these two electrode layers is formed so as to have a microscopically porous structure usually through the steps of applying a paste containing a powdered electrode material onto the surface of the shield layer or the solid electrolyte layer by a screen-printing technique, drying the resultant paste layer and firing the unfinished element to achieve sintering of the electrode material particles applied onto the aforementioned surface.
We have recognized that hitherto developed oxygen sensing elements of the above described laminated structure type are not yet fully satisfactory in their responsiveness, that is, the amount of time delay in responding to a change in the oxygen concentration in the gas in which the element is disposed, particularly when used in automotive engine exhaust systems, and that the responsiveness is significantly related to the physical structure of the measurement electrode layer formed through a firing process as mentioned above. The firing is performed at a considerably high temperature such as about 1500.degree. C. to achieve sufficient sintering of the electrode material applied onto the solid electrolyte layer surface by printing of a paste. Accordingly, there occurs considerable growth of the crystalline particles of the electrode material during the firing process, with the result that the measurement electrode layer is constituted of relatively coarse grains (in the microscopic sense) and therefore makes contact with a gas subject to measurement only in relatively small surface areas despite a porous structure of this electrode layer. In other words, the number and total area of so-called triple-phase points, where the solid electrolyte, measurement electrode and the gas come into contact with each other, provided by this measurement electrode layer are unsatisfactorily small relative to the macroscopic surface area of this electrode layer. By this reason, it takes a relatively large amount of time to establish an equilibrium oxygen partial pressure at the measurement electrode side of the solid electrolyte layer as the basis of generation of an electromotive force by the concentration cell, so that the oxygen sensing element does not very quickly respond to a change in the oxygen concentration in, for example, an engine exhaust gas resulting from a change in the air/fuel ratio of a gas mixture supplied to the engine.